Actually Paul, there is slight ambiguity in my spec, for which I apologise.

The original said "I have inserted a number of special code words into my Word documents, which always take the precise form ZXZABCDE... or ZXYZABCDE...

In detail - 1st char is always a capital Z. This is followed by precisely 1 or 2 capital alphabet characters (including Z), followed immediately by second capital Z (which can therefore be in position 3 or 4 only), followed by any number of alphabet characters (again, maybe including some Z's), all in caps, all one word."

I would perhaps now edit it as follows "I have inserted a number of special code words into my Word documents, which always take the precise form ZXZABCDE... or ZXYZABCDE...

In detail - 1st char is always a capital Z. This is followed by precisely 1 or 2 capital alphabet characters (which could be Z), followed immediately by capital Z (therefore position 3 or 4 must contain Z), followed by any number of alphabet characters (again, maybe including some Z's), all in caps, all one word.